What steps will reproduce the problem?
If possible, include a link to a program on play.golang.org.
1. Set up the preview go environment described here:
https://groups.google.com/d/msg/golang-dev/zHkWq3D92-4/5CI0bzTGO1sJ
2. Put this file on the model:

package main

import "math"

func main() {
    println(math.Log2(1))
}


3. go run log2.go

What is the expected output?

+0.000000e+000

What do you see instead?

+4.756597e-017

Which compiler are you using (5g, 6g, 8g, gccgo)?

gccgo.

Which operating system are you using?

Ubuntu Trusty.


Which version are you using?  (run 'go version')

4.8, 4.9.

Please provide any additional information below.

This is because gccgo uses a fused multiply add instruction in its compilation of
math.Log2.  Probably all of math (or even all of libgo) should be compiled with
-ffp-contract=off.